"No one could go to the actual course of the river" might be an ok translation. The cursus of the river is the bed or course in which it flows, not a specific portion of the river. The whole area was flooded, so the actual banks of the river were under water. I would translate ipsum by "actual" in idiomatic English, not "the same."

...but isn't the point that no one is likely to find the twins in this same place..and riverbed doesn't seem intuitive or even that idiomatic?? But that what's being referred to is the inaccessibility of THIS part or stretch of the river?

As I read the Latin, it says that the river was flooding, and no one could get to the actual bed or course of the river; so the slaves exposed the boys in a nearby marsh which had been created under the Palatine.

Better for "riverbed" in Latin is "alveus" or "canalis". The "cursus" is the "flow" or "course" or "direction of flow" that could potentially carry off what you throw in. The river's course sculpts and follows the riverbed but the riverbed won't carry off anything.

If you want to translate cursum as "the course of the river", that's fine. The point I was trying to make is that the river had overflowed its banks and the bed of the river -- the course in which it normally flowed -- was not accessible. I was trying to correct pmda's misunderstanding that cursum amnis meant a path to the river or a specific stretch of the river. It doesn't make as good sense in English at least to say that the course of the river was inaccessible due to flooding--it was the actual river bed that was inaccessible. Translate it however you want to.

Quimmik. Many thanks for your help (and your patience!). Actually my problem was that, for some reason, I was assuming that this was where the Tiber flowed into the sea and I was translating 'ripas' as beach. Don't ask me why. Now that you've explained it it makes perfect sense.